Giant Pressure Dependence and Dimensionality Switching in a Metal-Organic Quantum Antiferromagnet.

We report an extraordinary pressure dependence of the magnetic interactions in the metal-organic system [CuF_{2}(H_{2}O)_{2}]_{2}pyrazine. At zero pressure, this material realizes a quasi-two-dimensional spin-1/2 square-lattice Heisenberg antiferromagnet. By high-pressure, high-field susceptibility measurements we show that the dominant exchange parameter is reduced continuously by a factor of 2 on compression. Above 18 kbar, a phase transition occurs, inducing an orbital re-ordering that switches the dimensionality, transforming the quasi-two-dimensional lattice into weakly coupled chains. We explain the microscopic mechanisms for both phenomena by combining detailed x-ray and neutron diffraction studies with quantitative modeling using spin-polarized density functional theory.

Large Fermi Surface of Heavy Electrons at the Border of Mott Insulating State in NiS2.

One early triumph of quantum physics is the explanation why some materials are metallic whereas others are insulating. While a treatment based on single electron states is correct for most materials this approach can fail spectacularly, when the electrostatic repulsion between electrons causes strong correlations. Not only can these favor new and subtle forms of matter, such as magnetism or superconductivity, they can even cause the electrons in a half-filled energy band to lock into position, producing a correlated, or Mott insulator. The transition into the Mott insulating state raises important fundamental questions. Foremost among these is the fate of the electronic Fermi surface and the associated charge carrier mass, as the Mott transition is approached. We report the first direct observation of the Fermi surface on the metallic side of a Mott insulating transition by high pressure quantum oscillatory measurements in NiS2. Our results point at a large Fermi surface consistent with Luttinger's theorem and a strongly enhanced quasiparticle effective mass. These two findings are in line with central tenets of the Brinkman-Rice picture of the correlated metal near the Mott insulating state and rule out alternative scenarios in which the carrier concentration vanishes continuously at the metal-insulator transition.

Wigner Crystal and Colossal Magnetoresistance in InSb Doped with Mn.

We report magnetotransport investigation of nonmagnetic InSb single crystal doped with manganese at Mn concentration NMn ~ 1,5 × 10(17) cm(-3) in the temperature range T = 300 K-40 mK, magnetic field B = 0-25T and hydrostatic pressure P = 0-17 kbar. Resistivity saturation was observed in the absence of magnetic field at temperatures below 200 mK while applied increasing external magnetic field induced colossal drop of resistivity (by factor 10(4)) at B ~ 4T with further gigantic resistivity increase (by factor 10(4)) at 15T. Under pressure, P = 17 kbar, resistivity saturation temperature increased up to 1,2 K. Existing models are discussed in attempt to explain resistivity saturation, dramatic influence of magnetic field and pressure on resistivity with the focus on possible manifestation of three dimensional Wigner crystal formed in InSb by light electrons and heavy holes.

Brainwave counting.

Robert E. Ricklefs' name was misspelled throughout Roger Lewin's Research News artide "Egglaying" in birds remains a hot issue (29 Jan., p. 465).

The occurrence of radioactivity in public water supplies in the United States.

Examination of the collected data for radionuclide concentration measurements in public water supplies in the United States show more than 51,000 measurements for gross alpha-particle activity and/or Ra, 89,900 measurements for U, and 9,000 measurements for Rn. These measurements were made as part of national and state surveys of radionuclide concentrations in utility water supplies for Ra and Rn; and the National Uranium Resource Evolution (NURE) survey for U which included non-utility water supplies. Surface water has low values for Ra and Rn but levels comparable to ground water for U. Separate isotope measurements were not taken for much of the Ra and U data. Because 226Ra to 228Ra ratios and 238U to 234U ratios are not fixed in water, further measurements are needed to establish the specific isotopic concentrations by region. Analysis of the state average values in geological provinces shows the highest provincial areas for Ra are the Upper Coastal Plain, the glaciated Central Platform, and the Colorado plateau. For U, the highest areas are the Colorado plateau, the West Central Platform, and the Rocky Mountains. For Rn, the highest provinces are New England and the Appalachian Highlands-Piedmont. Regional hydrogeological and geochemical models are suggested for guiding the formulation of regional standards and monitoring strategies. Utility supplies serving small populations have the highest concentration for each radionuclide and have the lowest fraction of samples measured, which shows a need for further measurements of these small population water supplies. Risk estimates for the average concentration of Ra in utility ground water give about 941 fatal cancers per 70.7-yr lifetime in the United States. Risk estimates for the average concentration of U in utility surface and ground water give about 105 fatal cancers per 70.7-yr lifetime in the United States. Using 1 pCi/liter in air for 10,000 pCi/l in water, the Rn in utility water risk estimate is for 4,400-22,000 fatal cancers per 70.7-yr lifetime in the United States.